The invention is directed first to a sensor for measuring ionic activity by means of ion-selective membranes and associated shunts or leak conductances. Such an ion-selective membrane comprises a PVC base structure into which ionophori which are selected in accordance with the ions to be measured are introduced in determined quantitative ratios. Ionophori (called in German Ionophore) is used here as the plural of "ionophorus". Such ionophori are chemical compounds of a very particular structure which are known from the prior art. According to the quantity of active (free) ions, a certain phase boundary potential is formed at the membrane and toward the shunt contacting the membrane. This phase boundary potential can be viewed as the electromotive force (EMF) of an electrochemical half cell and is a measurement for the measured ionic activity. Ionometric sensors are known for carrying out serum and whole blood analysis in the area of a patient being tested bedside monitoring and in the laboratory. The blood, whose ionic activities are to be measured, is removed as a specimen (i.e., invasively) and introduced in the ionometer, wherein it is conducted past a series of membranes, (i.e., a flow-type electrode). It is disadvantageous that such an ionometer is suitable only for the very particular application described above, and that a determined quantity of whole blood (at least 2 to 3 ml) must be drawn from the patient for each measurement. This is particularly stressing for anemic (dialysis) patients and children. Moreover, there is an acute risk of infection, in principle, for the personnel every time blood is removed (particularly hepatitis B and AIDS).